The present invention relates to a waveguide-type optical device, for example, a waveguide-type optical device for light modulating or light switching using optic effects.
For a light modulator or a light switch, wide band, very high speed and low power operation is required. Accordingly, in theory, an electro-optic effect of which operation speed is high is used in many cases.
This electro-optic effect is a phenomenon that refractive index of a material changes when the material is charged by electric field. This effect is used for developing various kinds of waveguide-type optical devices.
FIG. 10 and FIG. 11 are figures showing configuration of a Mach-Zehnder light modulator as an example of a waveguide-type device using such electro-optic effects.
FIG. 10 is a plan view of the Mach-Zehnder light modulator and FIG. 11 is a cross section view taken on line II-II' of FIG. 10.
First, the configuration and operation of this light modulator are explained, referring to these pictures.
The Mach-Zehnder light modulator comprises the base 101 having an electro-optic effect, the light waveguide 102 formed on this base 101, the buffer layer 103 formed on the base 101 and the signal electrode 104 formed on this buffer layer 103.
Various kinds of material can be used for each component. For the base 101, for example, lithium niobate (LiNbO.sub.3) is used frequently. In this case, the light waveguide 102 is formed by thermally diffusing titanium into the LiNbO.sub.3 forming the base.
Moreover, a film made of silicon dioxide (SiO.sub.2) is used frequently for the buffer layer 103, also gold (Au) is used frequently for the signal electrode 104. The signal source for modulation 105 is connected with the signal electrode 104.
Next the operation of this light modulator is explained.
When the light from a semiconductor laser entries into light waveguide 102 from the light incident terminal 106, the incident laser light is divided into the light waveguides 102.sub.1 and 102.sub.2 at the light divergence section 107.
Here, if charging modulation voltage on the light waveguide 102.sub.1 using the signal source for modulation 105, a phase difference between the light propagating in the light waveguide 102.sub.1 and the light propagating in the light waveguide 102.sub.2 is caused by an electro-optic effect.
When these lights join at the light junction section 108, an interference corresponding to the phase difference arises. As the result, strength modulation corresponding to the modulation voltage is added to the light radiated from the light outgoing terminal 109.
For example, if charging modulation voltage to two lights alternately so as to make the phase difference between them become 0 and .pi., the light signal of which light strength alternately changes from maximum value and to minimum value is obtained at the light outgoing terminal 109.
In many cases, a traveling-wave-type electrode is used for such a light modulator to realize band widening or very high speed. In other words, this is a configuration that the micro wave modulation voltage is entered from an end of the signal electrode 104 that is a side of light entry and another end of the signal electrode is terminated using impedance of a signal line.
For designing such a large traveling wave electrode, it is necessary to arrange so as to obtain a large light modulation band .DELTA.f and enable to modulate the light with a small driving voltage v.pi..
Among them, the light modulation band .DELTA.f is dependent on the length 1 of signal electrode and the transmission refractive index n.sub.m of the micro wave. When a loss of the micro wave propagating in the traveling wave electrode is ignored, the relation among them is shown by the following equation. EQU .DELTA.f=1.9 C.sub.0 /(F(.pi.l.vertline.n.sub.m -n.vertline.))
Where, C.sub.0 is a velocity of light in vacuum, n is a refractive index of a light waveguide.
As shown by the equation, the light modulation band .DELTA.f (upper limit of a high speed modulation frequency) is limited by a difference between n.sub.m and n, that is, the closer .vertline.n.sub.m -n.vertline. comes to `0`, the larger light modulation band .DELTA.f.
By this reason, as described in the paper titled "Optimization of Ti:LiNbO.sub.3 modulator electrodes using finited element method" of the technical report of the Optical Quantum Electronics Department of the Institute of Electronics and Communication Engineers, No. 66, vol. 88 on May 30, 1988, the following parameters are determined when designing a signal electrode so as to make the transmission refractive index n.sub.m come close to the refractive index n of the light waveguide, based on the dielectric constant of the base used at computer simulation.
(1) Basic structure of a signal electrode
(2) Width W of signal electrode
(3) Interval G between signal electrodes
(4) Thickness t of a signal electrode
FIG. 12 shows a simulation result of a transmission refractive index n.sub.m that is obtained in the conditions that the basic structure of the signal electrode is a CPW (co-planar waveguide) structure, lithium niobate is used for the base material and the width W of signal electrode is 5 .mu.m. This figure shows a relation between the interval G of signal electrodes and the transmission refractive index n.sub.m when the thickness t of the signal electrode is a parameter.
The refractive index n of a light waveguide made by doping titanium on a base of lithium niobate is approximately 2.2. Accordingly, it is expected that the transmission refractive index n.sub.m of micro wave is also near by this value. As clearly known in the figure, there are many possible combinations of the interval G between signal electrodes and the thickness t of the signal electrode that makes the transmission refractive index n.sub.m be 2.2, that is, one of the two factors must be set firstly to determine the value of another factor.
The interval G between signal electrodes relates to the driving voltage V.pi., the narrower the interval G between signal electrodes, the smaller the driving voltage V.pi..
Also, from the view point of characteristics of a light modulator such as quenching ration, the narrower interval G between signal electrodes is better. In many cases, the value is set less than 15 .mu.m. By the reason, if the interval G between signal electrodes is set to 15 .mu.m, the thickness t of signal electrode is determined as 2 .mu.m by relation shown in FIG. 12.
As mentioned above, if each parameter of the signal electrode is determined the characteristic impedance Z of the signal electrode is decided from the characteristics such as the light modulation band .DELTA.f of a light device.
FIG. 13 shows a relation between the characteristic impedance Z and the interval G between signal electrodes.
FIG. 13 shows that if letting the interval G between signal electrodes be 15 .mu.m and the thickness t of the signal electrode be 12 .mu.m, the characteristic impedance Z becomes 46 .OMEGA..
In other words, if driving this device using the signal source for modulation of which characteristic impedance is 46 .OMEGA., a desired characteristic can be obtained.
The characteristic impedance of a signal source for modulation is, however, 50 .OMEGA. usually.
As it is difficult to design as the impedance and the transmission refractive index n.sub.m are set to desired values, the characteristic refractive index n.sub.m is firstly matched to the refractive index of a light waveguide in the conventional design stage for a waveguide-type optical device.
In addition, the impedance of the signal electrode of the obtained optical device is different from the impedance of the signal source for modulation. As the result, the conventional waveguide-type optical device has been used in a state that the characteristic impedance of the signal electrode and the characteristic impedance of the signal source for modulation does not coincided each other.
As shown in the Japanese Patent Publication No. 4-76456 (1992), there is a method to adjust the characteristic impedance Z of the signal electrode by adjusting thickness of a buffer layer. In this case too, however, there is a difficulty to design so that the impedance and the transmission refractive index n.sub.m are set to desired values as described before.
Described as above, a prior art has problems such as the device can not work fully because un-matching of impedance causes deterioration of modulation band or power reflection at the phase of signal input.